1. Field of the Invention
The present invention relates to telephone sets including a speech amplifier for transmitting on a loud-speaker the voice of the correspondent.
2. Discussion of the Related Art
FIG. 1 schematically shows a simplified architecture of a telephone set with a speech amplifier. A telephone line (not shown) is connected to an active terminal E and to a ground terminal G. The voltage available between terminal E and ground is referenced V.sub.L and the line current is referenced I.sub.L.
The telephone set comprises a template and supply circuit 10 for an amplifier 12 connected to a loud-speaker 13. Template circuit 10 is designed to establish a standard characteristic V.sub.L =f(I.sub.L) of line voltage V.sub.L versus line current I.sub.L. The current consumed to establish this characteristic is used to supply amplifier 12.
Circuit 10 comprises a voltage to current converter 15 having a transconductance G, supplied by the line voltage V.sub.L, and injecting a current I.sub.0 into a capacitor C connected to ground and into amplifier 12. The input voltage of converter 15 is established to a value V.sub.L -V.sub.1 by a constant voltage source V.sub.1 connected between terminal E and the input of converter 15.
This configuration achieves, on the one hand, a supply current I.sub.0 of amplifier 12 increasing as a function of the line voltage V.sub.L and, on the other hand, the characteristic V.sub.L =f(I.sub.L) of the template. ("Amplifier supply current" is to be construed as the sum of the currents flowing through amplifier 12 and capacitor C).
The parameters of the template circuit 10 (V.sub.1, G) are selected to account for currents consumed by other elements of the telephone set. These other elements comprise a current modulator 16 and additional circuits 17. Additional circuits 17, as for example a microprocessor, consume a constant current I.sub.1. Current modulator 16 serves to modulate the line current I.sub.L as a function of a speech signal V.sub.m. The current flowing through the modulator is referenced I.sub.m and has a d.c. component I.sub.m0. Thus, at any time, there is a mean line current I.sub.L =I.sub.0 +I.sub.m0 +I.sub.1. Within this disclosure, a current modulator is defined as a device which adds an AC component to a DC current, wherein the AC component is proportional to a voltage input to the device.
FIG. 2 illustrates a particular embodiment of a template circuit 10 using current mirror circuitry which is particularly suitable for integrated circuit design. Circuit 10 comprises a current mirror M1 connected to terminal E and receiving at its input a reference current i.sub.0, which is low as compared to line current I.sub.L. Current mirror M1 is a multiplier by a factor K, which factor is high with respect to 1 (approximately 40) and provides the supply current I.sub.0 (=Ki.sub.0) for amplifier 12. Reference current i.sub.0 is drawn by a current generator 18, generally a shunt regulator, comprising a voltage to current converter 19 having a transconductance g. Converter 19 is connected to a voltage source V.sub.1 in the same configuration as source V.sub.1 and converter 15 of FIG. 1. This configuration is equivalent to circuit 10 of FIG. 1 with Kg=G. Elements of such circuits are described in the data sheet of the SGS Thompson Microelectronics TEA 7063 Speech Circuit With Power Management Device.
In practice, in order to have modulator 16 modulating line current I.sub.L with a sufficiently high amplitude, it needs a quiescent current i.sub.m0 of approximately 5 mA. In addition, current I.sub.1 consumed by the additional circuits 17 is approximately 5 mA.
The available line current I.sub.L can, according to the nature and length of the line, have a low value, approximately 15 mA only; hence, only a low current (5 mA) is available to supply amplifier 12 (not taking into account a current close to i.sub.0 used by the template circuit 10 and not provided to the amplifier).
PCT application 8,503,614 describes a particular architecture of a telephone set having a speech amplifier, wherein the line modulation current is exploited to supply the speech amplifier, which adds the quiescent current of the modulator to the supply current of the speech amplifier.
However, the solution used in the above PCT application for exploiting the modulation current is not applicable to architectures using current mirrors, such as that of FIG. 2.